Discussion
Given below are two statements :
Statement I : A dc series motor is not suitable for line shaft drives.
Statement II : The Torque armature current characteristics for a dc series motor is of the form of a parabola.
In the light of the above statements, choose the most appropriate answer from the options given below.
Answer: D
Both Statement I and Statement II are true.
Statement I is true: a dc series motor's speed rises sharply as its load falls, and on no load it can race to a dangerous speed. Line shaft drives are belt drives, where the load can be lost if a belt slips or breaks, so series motors are not used for them; shunt motors are.
Statement II is true: in a series motor the field flux is produced by the armature current, so \(\phi \propto I_{a}\) and the torque \(T \propto \phi I_{a} \propto I_{a}^{2}\) before saturation. The torque against armature current curve is therefore a parabola (becoming nearly straight once the field saturates).
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